The Prime Focus Infrared Microlensing Experiment (PRIME): First Results

TL;DR

PRIME is a pioneering near-infrared microlensing survey telescope in South Africa, aiming to study planetary formation, measure free-floating planet masses via space-based parallax, and support transient event observations.

Contribution

It is the first dedicated NIR microlensing survey with a wide field of view, providing unique data on planetary populations and enabling mass measurements of free-floating planets.

Findings

Identified 486 microlensing candidates and over a thousand variable stars.

Established real-time alert system for follow-up observations.

Conducted initial survey since February 2024 with promising results.

Abstract

We present the description of the instruments and the first results of the PRime-focus Infrared Microlensing Experiment (PRIME). PRIME is the first dedicated near-infrared (NIR) microlensing survey telescope located at the South African Astronomical Observatory (SAAO) in Sutherland, South Africa. Among its class, it offers one of the widest fields of view in the NIR regime. PRIME's main goals are (1) To study planetary formation by measuring the frequency and mass function of planets. In particular, we compare results from the central Galactic bulge (GB), accessible only in the NIR by PRIME, with those from the outer GB by optical surveys. (2) To conduct concurrent observations with NASA's Nancy Grace Roman Space telescope. Due to the different lines of sight between the ground and space, we detect slight variations in light curves, known as ``Space-based parallax." This effect allows us to measure the mass of lens systems and their distance from the Earth. It is the only method to measure the mass of the free-floating planets down to Earth-mass. We began the GB survey in February 2024 and analyzed images through June 1, 2025, identifying 486 microlensing candidates and over a thousand variable stars, including Mira variables, which are useful to study the Galactic structure. We issue real-time alerts for follow-up observations, supporting exoplanet searches, and the chemical evolution studies in the GB. During the off-bulge season, we conduct an all-sky grid survey and Target of Opportunity (ToO) observations of transients, including gravitational wave events, gamma-ray bursts, and other science.